Sheet feeder and image forming apparatus having said sheet feeder

ABSTRACT

Disclosed is a sheet feeder in which sheets are fed by a feeding member from a tray on which the sheets have been loaded. When the tray is driven by a drive unit to lift up the tray from a sheet replenishment position to a position at which sheets can be fed, a first sensor senses that the top sheet on the tray is situated at a prescribed position. Response to this sensing of the first sensor the drive unit performs a predetermined intermittent driving for lifting the tray. It is determined based upon outputs from the first and second sensors whether or not a sheet is present on the tray.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet feeder for feeding sheets, and to an image forming apparatus having this sheet feeder.

2. Description of the Related Art

An image forming apparatus such as a printer, copier or facsimile machine is provided with a sheet feeder for feeding sheets to an image forming unit. The sheet feeder accommodates a plurality of sheets in stacked form on a liftable sheet loading member provided in a sheet accommodating section capable of accommodating the sheets, and feeds the sheets by a sheet feeding mechanism after the sheet loading member has been moved (lifted) to a position at which the feeding of sheets is possible. In such a sheet feeder, the sheet accommodating section is capable of being pulled out of the sheet feeder so as to facilitate the loading of sheets in the sheet accommodating section. It is so arranged that when the sheet accommodating portion is pulled out, the sheet loading member is lowered to the prescribed sheet loading position automatically. In particular, with a sheet feeder connected to a printer, a sheet surface sensor is provided for sensing the height of the top sheet of the sheets loaded on the sheet loading member. On the basis of information from the sheet surface sensor, the position of the sheet loading member is controlled in such a manner that the position of the top sheet will always be at a substantially fixed position.

In the sheet feeding mechanism, a pick-up roller is rotated in abutting contact with the top sheet when a sheet feed signal is sent from the image forming apparatus proper. The top sheet is fed next up to a pair of separation rollers. The separation rollers separate, sheet by sheet, the sheets fed to it by rotation of the pick-up roller and send the separated sheets into the image forming apparatus proper. When the pick-up roller sends sheets into the pair of separation rollers, the pick-up roller is withdrawn above the sheets out of contact with the sheets so as not to be an obstruction when the separation rollers separate the sheets. The sheet feeder repeats this operation whenever it is sent the sheet feed signal, thereby feeding the sheets to the image forming apparatus proper one sheet at a time.

In order to prevent the non-feeding of sheets or the feeding of overlapping sheets in such a feeding mechanism, it is preferred that the position of the top sheet on the sheet loading member be kept at a position where the pick-up roller will contact the top sheet with an appropriate feed pressure and can feed the sheet to the nip of the pair of separation rollers reliably. However, there are cases where a driving motor is used as the driving source that lifts the sheet loading member. Even if it is attempted to halt rotation of the driving motor in such cases, rotation cannot be halted immediately owing to inertial force (so-called overrun). This causes a variation in the position of the top sheet on the sheet loading member. In particular, when the sheet loading member is moved with a small number of sheets loaded on it, the driving force of the driving motor is too large relative to the weight of the sheets on the loading member. Consequently, the force of inertia also increases and there is a major change in the position of the top sheet.

As a method of solving this problem, it has been proposed to lift the sheet loading member up to a position at which feed is possible, then lower the sheet loading member temporarily and then reduce the voltage applied to the driving motor to thereby lift the sheet loading member again. As a result, overrun of the driving member can be reduced and the variation in the position of the top sheet on the sheet loading member can be reduced [see the specification of Japanese Patent Application Laid-Open No. 9-086680 (Patent Document 1)].

In accordance with Patent Document 1, the sheet loading member is lowered temporarily and then the voltage applied to the driving motor is reduced to drive the motor so as to lift the sheet loading member again. This expedient not only complicates control of movement but also leads to an increase in cost because it is necessary to provide circuitry that makes it possible to rotate the motor back and forth and for reducing the voltage applied to the motor. Further, since it is necessary to lift the sheet loading member twice, another problem is prolongation of the time it takes for the feeding of a sheet to become possible.

Further, in a proposal set forth in the specification of Japanese Patent Application Laid-Open No. 4-159932 (Patent Document 2), a timer for measuring the raising or lowering time of a hopper is provided. If a microswitch for sensing the position of a form in a prescribed period of time is not sensed, the rotation of a motor for raising or lowering the hopper is halted, after which the motor is driven intermittently. As a result, in cases where a large number of sheets of paper have been loaded and overrun therefore is small, the time needed to drive the motor is shortened by raising or lowering the hopper without driving the motor intermittently. In cases where a small number of sheets of paper have been loaded and overrun therefore tends to occur, the hopper is raised or lowered immediately without driving the motor intermittently up to a prescribed period of time that has been set. By then driving the motor intermittently upon elapse of this period of time, the hopper can be raised or lowered in a shorter time and overrun reduced.

With the arrangement described in Patent Document 2, measurement of the time needed to lift the hopper becomes impossible if the power supply is turned off during the raising of the hopper. This means that hopper overrun cannot be prevented reliably. Further, although lift-up time measured by the timer can be stored on a storage medium (e.g., an EEPROM) that is capable of retaining data even if power is cut off, this necessitates the provision anew of a storage medium or the like and leads to higher cost. An additional problem is more complicated control since processing for storing time on the storage medium constantly is required.

The specification of Japanese Patent Application Laid-Open No. 1-214528 (Patent Document 3) discloses a paper feeding mechanism having a table on which cut sheets of paper are placed; a table moving mechanism for moving the table up and down; and an upper-limit-position detecting mechanism comprising a lever, light-shielding plate and photo-interpreter for detecting the upper-limit position of the cut sheets of paper on the table. In accordance with this disclosure, the table rises and the lever is pushed upward by the cut sheets of paper, as a result of which the optical path of the photo-interpreter is blocked by the light-shielding plate and the motor raising the table is turned off. The light-shielding plate is formed to include a plurality of slits. As the table rises, the optical path of the photo-interpreter is blocked by the portions between the plurality of slits in the shielding plate. When this occurs, the motor is turned off and drive for elevating the table is halted. However, owing to inertia of the motor when the motor is stopped, a slit in the shielding plate reaches a position opposite the photo-interpreter, the motor turns on again and the table is lifted up. In other words, as a result of repetition of the on-off operation of the motor, the speed at which the table rises is slowed. Eventually, a slit for continuous feeding of paper comes into position opposing the optical path of the photo-interpreter and the table comes to rest.

With the arrangement of Patent Document 3, cost rises owing to provision of a special-purpose detection mechanism for controlling movement of the table. Further, since space is required in order to install the special-purpose detection mechanism, the apparatus is increased in size.

SUMMARY OF THE INVENTION

An aspect of the present invention is to eliminate the problems set forth above.

According to an aspect of the present invention, there is provided a sheet feeder comprising;

a tray on which sheets are loaded;

a feeding member configured to contact and feed a top sheet loaded on the tray;

a drive unit configured to lift the tray;

a first sensor configured to sense that the top sheet on the tray lifted by the drive unit is situated at a prescribed position;

a second sensor configured to sense that the top sheet on the tray lifted by the drive unit is situated at a feed position, at which feed is possible by the feeding member, higher than the prescribed position; and

a controller configured to control the drive unit so that the drive unit performs a predetermined intermittent driving for lifting the tray response to a sensing of the first sensor when the tray is lifted by the drive unit, and determine whether or not a sheet is present on the tray based upon the sensed states of the first and second sensors.

According to an aspect of the present invention, there is provided an image forming apparatus comprising: the sheet feeder set forth in claim 1; and

an image forming unit configured to form an image on a sheet transported from the sheet feeder.

Further features and aspects of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 depicts an external perspective view illustrating a laser printer according to an exemplary embodiment of the present invention;

FIG. 2 depicts an external perspective view illustrating a paper feed cassette in a state extracted from the laser printer in the state of FIG. 1;

FIG. 3 depicts a schematic sectional view illustrating the overall configuration of the laser printer fitted with a sheet accommodating unit according to the exemplary embodiment;

FIG. 4 depicts a mechanism view illustrating the details of a sheet feeding section of the paper feed cassette;

FIG. 5 is a diagram useful in describing a mechanism for lifting up a tray of the paper feed cassette;

FIG. 6 depicts an external view of the mechanism illustrating the tray in a state lowered from that shown in FIG. 4;

FIG. 7 is a diagram useful in describing a state in which the top sheet in a paper feed cassette is fed;

FIGS. 8A and 8B are diagrams useful in describing lift-up of the tray in the paper feed cassette according to this exemplary embodiment;

FIGS. 9A and 9B are diagrams useful in describing a state in which the tray has been lifted to make feeding possible;

FIG. 10 is a block diagram illustrating the configuration of a laser printer according to this exemplary embodiment;

FIG. 11 is a flowchart for describing control when lifting a tray, which is a sheet loading base, in the laser printer according to this exemplary embodiment;

FIG. 12 is a diagram for comparing control of a lift-up operation according to this embodiment and an ordinary lift-up operation based upon number of sheets in a cassette and amount of overrun; and

FIG. 13 is a timing chart for describing control of the lift-up operation in an laser printer according to this exemplary embodiment.

DESCRIPTION OF THE EMBODIMENT

A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that the following embodiment does not limit the present invention set forth in the scope of the claims and that all combinations of the features described in the embodiment are not necessarily essential as the inventive means for solving the foregoing problems.

An embodiment of the present invention will be described taking as an example a sheet accommodating unit connected to a laser printer.

FIG. 1 depicts an external view illustrating a laser printer 1 according to an embodiment of the present invention.

The laser printer 1 has a cassette 2 accommodating sheets S in stacked form. The cassette 2 is a single stage only and is provided within the laser printer 1. A three-stage cassette deck, which constitutes a sheet accommodating unit 51, serves also as a base on which the laser printer 1 is placed and is disposed below the laser printer 1. The sheet accommodating unit 51 is provided with paper feed cassettes 52 a, 52 b and 52 c. The paper feed cassettes 52 a, 52 b and 52 c accommodate, in stacked form, multiple sheets of respective ones of various sizes and weights. Casters are attached to the bottom of the sheet accommodating unit 51 at the four corners thereof in view of the fact that the laser printer 1 is moved while placed on the unit 51. Discharged sheets on which printing has been completed are stacked and accommodated in a discharge tray 12 with their printed side (image side) facing down.

FIG. 2 depicts an external perspective view illustrating the paper feed cassette 52 a pulled out of the laser printer 1 in FIG. 1.

The paper feed cassette 52 a has a sheet accommodating section provided with tray 101 on which sheets are loaded and which is free to move up and down within the cassette, and regulating plates 57, 58 and 59. The other paper feed cassettes 52 b and 52 c have a similar structure.

FIG. 3 depicts a schematic sectional view illustrating the overall configuration of the laser printer 1 fitted with the sheet accommodating unit. Portions in FIG. 3 similar to those shown in FIGS. 1 and 2 are designated by like reference characters.

A feed roller (pick-up roller) 3 serving as a feed member is rotated to feed sheets S, which have been loaded in the cassette 2, from the uppermost side. A pair of retard rollers 4 separate and transport, one at a time, the sheets S fed by rotation of the pick-up 3. A Sheet S thus separated one at a time is transported within the printer proper by transport rollers 5, 6.

A process cartridge 7 houses a well-known image forming process unit relating to image formation and is provided so as to be removable from the printer proper. The process cartridge 7 incorporates a photosensitive drum 7 a as an image carrier. A charging unit 7 b charges the surface of the photosensitive drum 7 a uniformly. The surface of the photosensitive drum 7 a charged by the charging unit 7 b is illuminated with laser light, which conforms to image information, from a laser exposure unit 8, whereby an electrostatic latent image is formed on the drum surface. A developing unit 7 c causes toner to attach to the surface of the drum 7 a on which the electrostatic latent image has been formed, thereby developing the electrostatic latent image into a toner image. A transfer roller 9 is brought into pressured contact with the photosensitive drum 7 a. When a sheet S transported by the transport rollers 6 passes between the photosensitive drum 7 a and the transfer roller 9, the toner image on the drum surface is transferred to the sheet S. A fixing unit 10 applies heat and pressure to the sheet S to which the image has thus been transferred, thereby fixing the transferred image on the sheet. The sheet S on which the image has thus been fixed is transported to a pair of discharge rollers 11. The sheet is discharged by rotation of the discharge rollers 11, image side down, onto the discharge tray 12 formed in the top side of the printer body. A cleaning unit 7 d recovers untransferred toner remaining on the surface of the photosensitive drum 7 a and cleans off the drum surface.

The mechanism for feeding sheets from the cassettes 52 a to 52 c provided in the sheet accommodating unit 51 will be described next. Since the three feed mechanisms are similar as well as the three cassettes, the description will be rendered taking the uppermost feed mechanism and the cassette 52 a as an example.

As illustrated in FIGS. 2 and 3, each paper feed cassette has a sheet accommodating section provided with the tray 101 on which sheet stack Sa is loaded and which is free to move up and down within the cassette, and regulating plates 57, 58 and 59. The mechanism has a pick-up roller 53 a, which is a sheet feeding roller for feeding a top sheet S1 of the sheet stack Sa placed on the tray 101. The mechanism further includes a pair of separation rollers formed by a feed roller 54 a and a retard roller 55 a for separating a sheet S fed by rotation of the pick-up roller 53 a. The mechanism further includes a transport roller 56 a for transporting the sheet S, separated and fed one sheet at a time by the separation rollers, to the image forming apparatus (printer) proper. It should be noted that pick-up rollers 53 b, 53 c, feed rollers 54 b, 54 c, retard rollers 55 b, 55 c and transport rollers 56 b, 56 c in the other paper feed cassettes 52 b, 52 c perform the same role as the similarly named rollers described above.

As illustrated in FIGS. 2 and 4, the tray 101 is connectably provided on a lifter drive unit 103 provided on the sheet accommodating unit 51 via a lift arm 102. The lifter drive unit 103 includes gears 111, 112, 113 and a lift motor 114, as illustrated in FIG. 5. FIG. 5 is a diagram useful in describing the lift-up mechanism of the tray 101 of paper feed cassette 52 a. When the lift motor 114 is driven into rotation, the speed of rotation is slowed down by the gears 111, 112, 113 and a gear 110 provided on the cassette 52 a and a semi-circular gear 109 connected to the lift arm 102 are rotated. As a result, the tray 101 is moved (lifted up) in the upward direction (in the direction of arrow A in FIG. 5).

FIG. 4 depicts a mechanism view illustrating the details of a sheet feeding section of a paper feed cassette 52 (the generic term for the cassettes 52 a to 52 c). Here the tray 101 is illustrated in the lifted state. In FIG. 4, rollers 53 to 56 correspond to the above-mentioned pick-up rollers 53 a to 53 c, feed rollers 54 a to 54 c, retard rollers 55 a to 55 c and transport rollers 56 a to 56 c, respectively.

FIG. 6 depicts an external view of the mechanism illustrating the tray 101 in a state lowered from that shown in FIG. 4.

FIGS. 8A and 8B are diagrams useful in describing lift-up of the tray 101 in a paper feed cassette according to this embodiment, and FIGS. 9A and 9B are diagrams useful in describing a state in which the tray has been lifted to make feeding possible. FIG. 9A illustrates a state in which sheets are present on the tray 101, and FIG. 9B a state in which there are no sheets on the tray 101.

A sheet detector 105 detects whether sheets have been loaded on the tray 101 and has a first photosensor 106 and a first flag 107 that is moved so as to be capable of turning on and off an infrared beam directed toward the first photosensor 106. One end of the first flag 107 abuts against and is pushed by the top sheet, whereby the first flag 107 is turned about its axis. The first flag 107 blocking the infrared beam of the first photosensor 106 is moved as a result, whereby the infrared beam is allowed to reach the first photosensor 106 [this state (detection of paper) is the “sensor on” state]. If there is no application of external force, the first flag 107 waits at a position to which it has fallen under its own weight (such a state is illustrated in FIG. 8A). At such time the infrared beam directed toward the first photosensor 106 is blocked by the first flag 107 [this state (no paper) is the T“sensor off” state].

The tray 101 is provided with a cut-away portion 108, as illustrated in FIG. 4. As a result, when the tray 101 is lifted without any sheets being loaded thereon, the first flag 107 does not contact the tray 101 and remains in the stand-by state under its own weight, as described above. Accordingly, in a case where the tray 101 has been lifted without sheets being loaded on the tray 101, the first photosensor 106 remains cut off from the infrared beam (sensor off) in the manner set forth above.

A sheet detection signal 1051 (FIG. 10), which indicates whether a sheet is present or not, from the sheet detector 105 changes over from the OFF state to the ON state (the state in which the first flag 107 abuts against a sheet) if a sheet has been placed at a prescribed position on the tray 101 in lift-up thereof.

A top-sheet detector 104 (FIGS. 4 and 10) includes a second flag 207 (see FIGS. 8A, 8B, 9A, 9B) placed above a sheet stack Sa placed upon the tray 101 and capable of being turned by coming into abutting contact with the top sheet of the sheet stack Sa, and a second photosensor 115 (see FIGS. 4, 6, 8A, 8B, 9A, 9B) turned on and off by the second flag 207. One end of the second flag 207 is pushed by coming into abutting contact with the top sheet, whereby the second flag 207 turns about its axis. When the second flag 207 moves, an infrared beam impinging upon the second photosensor 115 is blocked by the second flag 207. This state in which the second photosensor 115 is cut off from the beam is the “sensor off” state. If there is no application of external force, the second flag 207 waits at a position to which it has fallen under its own weight (such a state is illustrated in FIG. 8A). In this stand-by state, the infrared beam reaches the second photosensor 115 (this is the “sensor on” state).

The position at which the top-sheet detector 104 detects a sheet on the tray 101 is a position above that at which the sheet detector 105 detects a sheet. A top-sheet detection signal 1041 (FIG. 10) from the top-sheet detector 104 is turned off (feed becomes possible) when the top sheet on the tray 101 has been lifted to a position at which feed becomes possible by the pick-up roller 53.

If the tray 101 is lifted without sheets being loaded on the tray 101, the second flag 207 is contacted by the tray 101 and turns (see FIG. 9B). Owing to this movement of the second flag 207, the second photosensor 115 changes over from the sensor-on to the sensor-off state, as described above.

When the paper feed cassette 52 a is pulled from the sheet accommodating unit (deck) 51, as illustrated in FIG. 2, the tray 101 is released from its connection to the lifter drive unit 103 (FIG. 4) provided on the sheet accommodating unit 51. As a result, the tray 101 drops to the lowermost portion of the paper feed cassette 52 a under its own weight.

When the placing of the sheet stack Sa on the tray 101 has been completed, the paper feed cassette 52 a is set in the sheet accommodating unit 51 in the manner depicted in FIG. 1. A controller (1001 in FIG. 10) that has sensed this operation drives the lifter drive unit 103 into rotation, thereby lifting the connected tray 101. As described above, the first flag 107, which is capable of being turned by contact with the top sheet of the sheet stack Sa, and the first photosensor 106 turned on and off by the first flag 107 are provided on the upper side of the sheet stack Sa placed on the tray 101. Further provided is the top-sheet detector 104, which includes the second flag 207 capable of being turned by contact with the top sheet of the sheet stack Sa and the first photosensor 106 turned on and off by the second flag 207.

In a case where the top sheet on the tray 101 is at a position lower than a prescribed position, the second flag 207 in the top-sheet detector 104 moves under its own weight and leaves the position at which it blocks the light of the second photosensor 115, as a result of which the output of the second photosensor 115 attains the ON state.

If the tray 101 is thenceforth lifted by the lifter drive unit 103 so that the top sheet on the tray 101 is lifted to the prescribed position, the first flag 107 abuts against the top sheet. If the tray 101 is lifted further, then the second flag 207 is turned to a position at which it blocks the light of the second photosensor 115, whereby the output of the second photosensor 115 attains the OFF state. It is so arranged that the position at which the output of the second photosensor 115 attains the OFF state is a position at which the top sheet on the tray 101 can be fed by rotation of the sheet pick-up roller 53 and guided smoothly to the nip between pair of separation rollers 54, 55. Control is exercised in such a manner that the lifter drive unit 103 halts rotation when the top-sheet detection signal 1041 attains the OFF state (i.e., when the top sheet on the tray 101 is lifted to a position at which feed is possible). As a result, the top sheet on the tray 101 is always placed at a position where it is capable of being fed.

FIG. 7 is a diagram useful in describing a state in which the sheet in a paper feed cassette is fed. Portions in FIG. 7 similar to those shown in the above described drawings are designated by like reference characters.

When a sheet feed signal is sent from the image forming apparatus proper, the pick-up roller 53 comes into abutting contact with top sheet S1 and rotates, as indicated at 53-1. Owing to rotation of the pick-up roller 53, the top sheet is fed up to the pair of separation rollers 54, 55. The separation rollers 54, 55 separate and feed, one sheet at a time, sheets fed in by rotation of the pick-up roller 53 and send each sheet into the image forming apparatus. It is so arranged that when the pick-up roller 53 feeds a sheet to the pair of separation rollers 54, 55, the pick-up roller 53 is withdrawn above the sheet S1 out of contact with the sheet S1 so as not to be an obstruction when the separation rollers 54, 55 separate the sheets. Thus, whenever a sheet feed signal is sent from the image forming apparatus, the above-described operation is repeated so that sheets are transported to the image forming apparatus one sheet at a time.

In a state in which the tray 101 has been lowered to the lowermost position, as depicted in FIG. 8A, the first flag 107 is waiting, under its own weight, at the position shown in FIG. 8A. At this time the infrared beam directed toward the first photosensor 106 is being blocked by the first flag 107 (the first photosensor 106 is in the OFF state). On the other hand, the second photosensor 115 under these conditions is not cut off from light by the second flag 207 and is in the ON state.

FIG. 8A is for describing a lift-up zone in which the tray 101 is lifted up from its lowermost position by ordinary lift-up processing. FIG. 8B, on the other hand, illustrates a lift-up control interval in which the lift-up operation is controlled, this being performed in a prescribed zone in which the tray 101 is lifted up from its lowermost position to its uppermost position (feed position). The lift-up control interval will be described below in detail. When the sheet detector 105 changes over from the sensor-off state to the sensor-on state during lift-up, the lift-up operation is halted temporarily and lift-up control, described below, is executed, as illustrated in FIG. 8B.

FIGS. 9A and 9B are diagrams useful in describing a state in which the tray 101 has been lifted to make feeding possible. FIG. 9A illustrates a state in which sheets are present on the tray 101, and FIG. 9B a state in which there are no sheets on the tray 101.

With sheets loaded on the tray 101, the tray 101 is lifted up to the feed position by the lifter drive unit 103 described above. As the tray 101 is in the process of being lifted, one end of the first flag 107 abuts against the top sheet S1 and is pushed toward the upper side. As a result, the first flag 107 turns in the clockwise direction about a shaft 107 a. Consequently, as illustrated in FIG. 9A, the infrared beam of the second photosensor 115 is blocked and the second photosensor 115 is cut off from the beam (i.e., attains the OFF state).

If the tray 101 is lifted without sheets being loaded thereon, as illustrated in FIG. 9B, then the first flag 107 passes through the cut-away portion 108 (FIG. 4) in the tray 101. As a result, the first flag 107 does not contact the tray 101 and is in the standby state owing to its own weight, in a manner similar to that shown in FIG. 8A. Accordingly, the infrared beam directed toward the second photosensor 115 remains unobstructed while the infrared beam directed toward the first photosensor 106 remains in the blocked state. If the tray 101 is lifted without sheets being loaded thereon, the second flag 207, on the other hand, contacts the tray 101 and is turned (see FIG. 9B). Owing to such motion of the second flag 207, the second photosensor 115 changes over from the sensor-on state to the sensor-off state.

If the signal from the first photosensor 106 remains OFF when the tray 101 is lifted and the second photosensor 115 changes over to the OFF state, then a decision is rendered to the effect that there are no sheets placed on the tray 101. In this case, the feeding operation is inhibited and absence of paper is displayed on a console unit 1004 (FIG. 10), which is an indication unit on the image forming apparatus, whereby the operator, etc., can be prompted to replenish the sheets of paper.

FIG. 10 is a block diagram illustrating the configuration of the laser printer 1 according to this embodiment.

A controller 1001 controls the overall laser printer and includes a CPU 1010 such as a microprocessor, a ROM 1011 for storing programs, which are executed by the CPU 1010, and data, and a RAM 1012 for storing various data temporarily. A printer engine 1002 has the laser exposure unit 8 shown in FIG. 3, and various motors and the fixing unit 10 for forming an image following installation of the process cartridge 7. On the basis of a command from the controller 1001, a motor driver 1003 rotates the lift motor 114 to control lift-up of the tray 101. As mentioned above, the top-sheet detector 104 has the first flag 107 and second photosensor 115. When the top sheet on the tray 101 has been lifted by rotation of the pick-up roller 53 to the position at which feed is possible, the top-sheet detector 104 sends the top-sheet detection signal 1041 to the low level (OFF). As mentioned above, the sheet detector 105 has the first flag 107 and first photosensor 106. If a sheet has been placed at a prescribed position of the tray 101 in lift-up thereof, then the sheet detector 105 changes over the sheet detection signal 1051 from the OFF to the ON state. The console unit 1004 has a display for displaying messages to the user, and a control panel manipulated by the user. It should be noted that other sensors and mechanisms not referred to in this description of the embodiment are not illustrated in FIG. 10.

FIG. 11 is a flowchart for describing control when lifting the tray 101, which is a sheet loading base, in the laser printer 1 according to this embodiment. The program for executing this processing has been stored in the ROM 1011 and is executed under the control of the CPU 1010.

The processing illustrated in FIG. 11 is started by inserting the paper feed cassette 52 into the sheet accommodating unit 51, by way of example. Sheet cassette sensing means (not shown) senses the insertion of the cassette. Usually the top-sheet detector 104 is in the sensor-on state and the sheet detector 105 in the sensor-off state at this time, as illustrated in FIG. 8A. In step S1, the CPU 1010 determines whether the top-sheet detection signal 1041 is ON. Since the signal 1041 is ON, control proceeds to step S2, where the CPU 1010 determines whether the sheet detection signal 1051 is ON (sheets present). Since the sheet detection signal 1051 is OFF, control proceeds to step S3. Here the lift motor 114 is rotated via the motor driver 1003 and the CPU 1010 exercises control so as to perform the ordinary lift-up operation. Here “ordinary lift-up operation” refers to an operation in which the tray 101 is lifted by rotating the lift motor 114 continuously.

If the sheet detection signal 1051 from the sheet detector 105 changes over from the sensor-off state to the sensor-on state during this lift-up, as illustrated in FIG. 8B, control proceeds from step S2 to step S4. In step S4, the CPU 1010 controls drive of the lift motor 114 in such a manner that rotation of the lift motor 114 is halted to stop the lift-up operation for a given period of time. Here the lift-up operation is stopped for 100 ms, by way of example. Next, control proceeds to step 5S, where the CPU 1010 determines whether the top-sheet detection signal 1041 from the top-sheet detector 104 has turned OFF. When the top-sheet detection signal 1041 turns OFF, this means that the top sheet on the tray 101 has reached the feed position. If the top-sheet detection signal 1041 is still in the ON state, then control proceeds to step S6. Here the CPU 1010 controls drive of the lift motor 114 in such a manner that the lift motor 114 is rotated to perform the lift-up operation for 30 ms, by way of example. Control then proceeds to step S4. Here the rotation of the lift motor 114 is halted again for the prescribed period of time, control proceeds to step S5 and the above-described processing is repeated.

If the CPU 1010 determines at step S5 that the top-sheet detection signal 1041 has turned OFF, meaning that the top sheet of the tray 101 has reached the feed position, then control proceeds to step S7. Here the feed standby state illustrated in FIG. 9A is attained.

If the top-sheet detection signal 1041 is found to be OFF (i.e., the top sheet has reached the feed position) in step S1, on the other hand, then control proceeds to step S8, at which the CPU 1010 determines whether the sheet detection signal 1051 at this time is ON (paper present) or not. If the sheet detection signal 1051 is OFF, then this indicates that there are no sheets in the cassette and control therefore proceeds to step S9. Here a display indicating absence of sheets is presented on the console unit 1004 to warn the user. If the sheet detection signal 1051 is found to be ON (paper present) at step S8, then control proceeds to step S7 and the feed standby state illustrated in FIG. 9A is attained.

FIG. 12 is a diagram for comparing control of the lift-up operation according to this embodiment and the ordinary lift-up operation based upon number of sheets in a cassette and amount of overrun.

Control of the lift-up operation according to this embodiment is indicated at 1200, and control of the lift-up operation according to the prior art is indicated at 1201. It will be understood from this comparison that by exercising control according to this embodiment, the amount of overrun due to the lift-up operation can be kept small even in a state in which a small number of sheets are loaded in the cassette.

FIG. 13 is a timing chart for describing control of the lift-up operation in the laser printer according to this embodiment.

Rotation of the lift motor 114 has been started with the top-sheet detection signal 1041 in the ON state and the sheet detection signal 1051 in the OFF state (timing T1). At timing T2, the sheet detection signal 1051 changes from the OFF to the ON state, as a result of which control proceeds from step S2 to step S4 in FIG. 11 and rotation of the lift motor 114 is halted temporarily. In a lift-up control zone 1300, lift-up drive is performed repeatedly (S6) by rotating the lift motor 114 intermittently 30 ms at a time, as in the example described above. At timing T3, the top-sheet detection signal 1041 assumes the OFF state (“YES” at S5) and the top sheet on the tray 101 attains the state in which feed is possible, whereupon rotation of the lift motor 114 is halted to complete lift-up of the tray 101.

By exercising such control, the amount of overrun of the tray 101, in which the tray 101 is lifted up too much owing to the inertial force of the lift motor 114, can be reduced more by lift-up control 1200 according to this embodiment in comparison with ordinary lift-up control indicated at 1201, as illustrated in FIG. 12. As a result, the feed position at the time of feed is stabilized and problems such as non-feeding of sheets and feeding of overlapping sheets can be prevented.

Further, by using control according to this embodiment, the time required for lift-up is longer than when lift-up is performed in the ordinary manner. According to this embodiment, however, the zone in which intermittent drive is performed using the result of detection by the sheet detector 105 is limited and therefore the time is takes for lift-up also can be made comparatively short.

Further, even in a case where the power supply is turned off during lifting of the tray 101, the next time the power supply is turned on it can be determined by the sheet detector 105 in what state the tray 101 serving as the sheet loading base resides. This means that it is no longer necessary to store the conditions prevailing during lift-up, and the lift-up operation of the tray 101 can be controlled appropriately even if the power supply is turned off during lift-up.

In another conceivable arrangement, the quantity of sheets remaining in a cassette is detected using a residual-sheet detecting unit, which is for detecting quantity of sheets remaining on the tray 101, instead of the sheet detector 105. Then, when a prescribed residual sheet quantity is attained, lift-up control is executed. In this embodiment, a sensor serving as a trigger to start control and a sensor for detecting whether or not a sheet is present are combined. As a result, the number of component parts can be reduced as compared with the conventional arrangement provided with a special-purpose sensor for performing lift-up control. This makes it possible to hold down cost.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2006-180370, filed Jun. 29, 2006, which is hereby incorporated by reference herein in its entirety. 

1. A sheet feeder comprising; a tray on which sheets are loaded; a feeding member configured to contact and feed a top sheet loaded on said tray; a drive unit configured to lift said tray; a first sensor configured to sense that the top sheet on said tray lifted by said drive unit is situated at a prescribed position; a second sensor configured to sense that the top sheet on said tray lifted by said drive unit is situated at a feed position, at which feed is possible by said feeding member, higher than the prescribed position; and a controller configured to control said drive unit so that said drive unit performs a predetermined intermittent driving for lifting said tray response to a sensing of said first sensor when said tray is lifted by said drive unit, and determine whether or not a sheet is present on said tray based upon the sensed states of said first and second sensors.
 2. The sheet feeder according to claim 1, wherein said controller controls so that said drive unit stops a driving response to a sensing of said second sensor.
 3. The sheet feeder according to claim 1, further comprising a first flag configured to be moved by coming into contact with the top of a sheet loaded on said tray; wherein in a case where a sheet is present on said tray, said first flag changes over the sensed state of said first sensor in a case that the top sheet reaches the prescribed position owing to lifting of said tray.
 4. The sheet feeder according to claim 3, further comprising a second flag configured to be moved by coming into contact with the top of a sheet loaded on said tray or with said tray per se; wherein said second flag changes over the sensed state of said second sensor by coming into contact with the top sheet in a case that the top sheet reaches the feed position owing to lifting of said tray.
 5. The sheet feeder according to claim 4, wherein said controller determines that there are no sheets on said tray in a case that the sensed state of said second sensor has changed, without a change in the sensed state of said first sensor, owing to lifting of said tray.
 6. The sheet feeder according to claim 1, wherein said controller drives said drive unit continuously until said first sensor senses that said tray has reached the prescribed position.
 7. An image forming apparatus comprising: the sheet feeder set forth in claim 1; and an image forming unit configured to form an image on a sheet transported from said sheet feeder. 